I can "draw" the triangle in my mind, as you would with fingers in the air: I know the shape it would have made if I drew it on paper, but when drawing with my fingers in the air, my fingers leave no trail, and that's the same in my mind if I try to "project" something mentally.

I can draw a world map from memory with well above average accuracy, but I can't picture it even if I mentally trace the outline I would put on paper.